Aluminum reflector with composite reflectivity-enhancing surface layer

ABSTRACT

Reflector having a composite reflectivity-enhancing layer as reflecting surface layer on a reflector body where the said composite layer has an outer layer facing the radiation to be reflected, the HI-layer, with a refractive index n 2 , and, between the reflector body and the outer layer, an LI-layer with a refractive index n 1  which is smaller than n 2  and the LI and HI layers are λ/4 layers. The HI-layer is a sol-gel layer and the optical layer thickness d opt ,1 of the LI layer and d opt ,2 of the HI layer are such that 
     
         d.sub.opt,i =d.sub.i.n.sub.i =l.sub.i.λ/4±20 nm, i=1 or 2 
    
     where d 1  represents the thickness of the LI layer in nm, d 2  the thickness of the HI layer in nm, λ the average wavelength in nm of the light striking the reflector surface and l 1 , l 2  are uneven natural numbers. The LI layer is a barrier layer of aluminium oxide made by anodising, or a sol-gel layer.

This application is a divisional application of U.S. Ser. No.08/876,332, filed on Jun. 6, 1997, now U.S. Pat. No. 5,978,133.

The present invention relates to a reflector having a compositereflectivity-enhancing layer as reflecting surface layer on a reflectorbody where the said composite layer has an outer layer facing theradiation to be reflected, the HI-layer, with a refractive index n₂, andbetween the reflector body and the outer layer an LI-layer with arefractive index n₁ which is smaller than n₂ and the LI and HI layersare λ/4 layers. The invention also relates to the use of such reflectorswith reflectivity enhancing composite layer and to a process for itsmanufacture.

Reflectors featuring a composite layer system comprising LI/HI-layersdeposited on aluminium (LI/HI=Low Refraction Index/High RefractionIndex)--i.e. layers exhibiting an inner layer with refractive index n₁(LI) and an outer layer with a refractive index n₂ which is greater thann₁ --are in general known as reflectors with surface layers that enhancereflectivity.

Such reflectors are nonnally produced by depositing a very thin layer ofhigh purity aluminium onto the reflector body e.g. of glass or technicalgrade aluminium (i.e. aluminium of lower purity) e.g. by means of PVD(physical vapour deposition) methods such as sputtering or vaporisation.The high purity Al layer is then protected by depositing on it an LIprotective layer e.g. made of Al₂ O₃, or SiO₂, usually by PVD or CVD(chemical vapour deposition) methods, and enhanced by a further HI layerto provide a LI/HI reflectivity-enhancing surface on the reflector.

Because of the small thickness of the layer, it is generally notpossible to anodise PVD Al layers; consequently, the deposition of theLI and HI layers by PVD or CVD methods is normally carried out underhigh vacuum. In order to achieve high reflectivity characteristics withcomposite layers that improve reflectivity, it is necessary to achievegood homogeneity and to keep exactly to narrow, exactly pre-definedtolerances in the thickness of the individual layers. Keeping closely tothe exact thickness tolerances of oxide layers deposited in light vacuumusing PVD or CVD methods, and checking the thicknesses of these layersis difficult and requires complicated, expensive equipment. The rate ofdeposition of CVD or PVD layers, especially such dielectric layers,depends on the method used and--compared with chemical methods--isrelatively low. In view of the high cost of high-vacuum depositionunits, this leads to high manufacturing costs. Furthermore, the lowdeposition rates and the necessity to use high-vacuum equipment for thePDV or CVD processes makes it difficult or even impossible to producethe layers in a continuous manner.

A further possibility for manufacturing composite layers providingreflectivity-enhancing composite layers is to use chemical or anodicoxidation of aluminium surfaces and subsequently to deposit a dielectriclayer with a higher refractive index than aluminuim. For that purposeone reflectors made of aluminum or reflectors with a layer of aluminiumwhich is thick enough for anodising. Anodising is normally performed ina sulphuric acid electrolyte using direct current (dc anodising). Bychoosing the appropriate parameters the resultant LI layer can be ahomogeneous layer with predefined later thickness but normally exhibitshigh porosity which is a result of the process itself. The deposition ofthe HI layer is normally carried out using PVD or CVD methods. Suchreflectivity enhancing composite layers may be produced e.g. in a stripprocess.

The object of the present invention is the preparation, at favourablecost, of reflectors with reflectivity-enhancing composite layers, inparticular for technical lighting purposes, whereby the above mentioneddisadvantages of the state of the art reflectors are avoided and, inparticular, are suitable for continuous production in a strip process.

That objective is achieved by way of the invention in that the HI layeris a sol-gel layer and the optical layer thickness d_(opt),1 of the LIlayer and d_(opt),2 of the HI layer are such that

    d.sub.opt,i =d.sub.i. n.sub.i =l.sub.i. λ4 ±20 nm ,=1 or 2

where d₁ represents the thickness of the LI layer in nm, d₂ thethickness of the HI layer in nm, λ the average wavelength in nm of thelight striking the reflector surface and l₁, l₂, uneven natural numbers.

It must be taken into account that the refractive index n , i.e. n₁ orn₂, --because of the dispersion of the light--is a function of thewavelength i.e. in the present text ₁ and n₂ always refer to thecorresponding wavelength of the light striking the reflector surface.Furthermore, it must be taken into account that the condition d_(i).n_(i) =l_(i). λ4 , i=1 or 2 to obtain a reflectivity-enhancing compositelayer is completely valid only for electromagnetic radiation strikingthe reflector surface vertically.

The LI/HI multiple layers are usually made up of at least two layerswith different refractive indices. The combination of a pair ofdielectric layers of different refractive index on a metal surface--inwhich the layer with the lower refractive index is situated on thesurface of the reflector body--allows better reflectivity properties tobe obtained than with a single horngeneous layer. For a given layercomposition the highest reflectivity can be achieved if the opticallayer thickness of the individual layers amounts to λ/4 or an unevenmultiple thereof. With respect to the composition of the layermaterials, the best reflectivity characteristics are achieved when thedifference in the refractive indices of the individual layers is asgreat as possible.

The reflector according to the invention, which exhibits an HI sol-gellayer as an essential feature of the invention, offers advantages overthe known, state of the art reflectors in that such HI layers can bedeposited economically with the required constant thickness, theiradhesion to the LI layer can be optimised readily e.g. by choice of theappropriate crosslinking agent i.e. without restricting the freedom ofchoice of other components that determine the refractive index of the HIlayer, a large range of commercially obtainable, highly transparentsol-gel layers is already available, and the sol-gel layers in generalexhibit very good behaviour with respect to levelling out the surface.

By properly choosing the composition of the sol for the sol-gel layer,in particular the crosslinking agent, the viscosity of the sol can bereadily optimised for a given thickness of sol-gel layer. Furthermore,in general, sol-gel layers exhibit good resistance to scratching andgood formability, it being possible to optimise these properties by thechoice of composition of the sol-gel layer. In general, sol-gel layerscan also be deposited readily using PVD, which also allows any otherdesired layers to be deposited on die free surface of the HI layer--e.g.semi-transparent layers.

A significant advantage of the reflectors according to the invention is,however, the replacement of PVD or CVD HI layers by sol-gel HI layers,as a result of which the reflectors according to the invention can beproduced completely on cost-fivourable strip-coating units.

The reflector body required for the reflector according to the inventionis preferably of pure aluminium or an aluminium alloy.

The aluminium body may be part of a component, e.g. a section, beam oranother form of components, a plate, strip, sheet or a foil ofaluminium, or may be an aluminium outer layer of a composite material,in particular an aluminium outer layer of a composite panel, or analuminium layer deposited e.g. electrolytically on any material ofchoice. In a preferred version, the reflective body bearing thealuminium layer concerns a component made of aluminium which has beenmanufactured e.g. by rolling, extrusion, forging or press-forming. Thereflector body containing the aluminium layer may also be shaped bybending, deep-drawing, cold press-forming or the like.

In the present text the term aluminium is to be understood to includeall grades of purity of aluminium and all commercially availablealuminium alloys. For example, the term aluminium includes all rolling,wrought, casting, forging and extrusion alloys of aluminium. Usefully,the aluminium layer is of pure aluminium having a purity level of 98.3wt. % or more or aluminium alloys made with this purity grade andcontaining at least one of the elements Si, Mg, Mn, Cu, Zn or Fe. Thealuminium layer of pure aluminium exhibits e.g. a purity of 98.3 wt. %and higher, usefully 99.0 wt. % and higher, preferably 99.9 wt. % andhigher, especially 99.95 wt. % and higher.

Besides aluminium of the above mentioned purities, the aluniiniumn layermay also contain 0.25 to 5 wt. %, especially 0.5 to 2 wt. % magnesium,or 0.2 to 2 wt. % manganese, or 0.5 to 5 wt. % magnesium and 0.2 to 2wt. % manganese, especially e.g. 1 wt. % magnesium and 0.5 wt. %manganese, or 0.1 to 12 wt. %, preferably 0.1 to 5 wt. % copper, or 0.5to 5 wt. % zinc and 0.5 to 5 wt. % magnesium, or 0.5 to 5 wt. % zinc,0.5 to 5 wt.9) magnesium and 0.5 to wt. % copper, or 0.5 to 5 wt. % ironand 0.2 to 2 wt. % manganese, in particular e.g. 1.5 wt. % iron and 0.4wt. % manganese.

The surface of the reflector body may have any desired shape and may, ifdesired, be also be structured. In the case of rolled reflector bodysurfaces, these may e.g. be treated by high gloss or designer rolls. Apreferred application for structured reflector body surfaces is e.g. forreflectors for daylight lighting where in particular structured surfacesexhibiting structural features of the order of 0.1 to 1 mm are employed.

Essential to the invention is that the HI layer is a sol-gel layer.Sol-gel layers are understood here as are layers manufactured by asol-gel process. Sol-gel layers are e.g. xero-gels. Preferred for theproduction of the sol-gel layers required for the reflectors accordingto the invention are lyosols, whereby lyosols may be organosols orlydrosols. Preferred are organosols. The gel layer--in the followingsol-gel layer--required for the reflectors according to the invention isformed e.g. by coagulation.

Also preferred for the production of the sol-gel layers are sols i.e.colloidal solutions, in which one of the following oxides or a mixtureof the following oxides is dispersed in a fully divided form in a fluidmedium.

The HI layer preferably comprises or contains oxides of alkali mealse.g. Li, alkaline earth metals e.g. Mg, Ca, Sr, Ba and/or transitionmetals such as e.g. Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Y, Zr, Nb, Mo, Te,Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir, Pt and/or lantianides such as e.g.La, Cc, Pr, Nd, Pm, Dy, Yb or Lu, etc. Preferred for the reflectorsaccording to the invention, featuring III layers deposited in a sol-gelprocess, are HI layers of essentially titanium oxide (Ti-oxide) with arefractive index of approx. 2.5, praseodymium-Litanium oxide(PrTi-oxide), lanthanum-titanium oxide (LaTi-oxide), tantalum oxide(Ta-oxide), hafnium oxide (Hf-oxide), niobium oxide (Nb-oxide),Zin-oxide, Ce-cxide or an oxide of an alloy of the mentioned materials.Especially preferred are, however, HI layers comprising or containingTiO₂, Ta-oxide, PrTi-oxide or LaTi-oxide.

The Sol-gel layers are glassy in character. Sol-gel layers can bemanufactured reproducibly with a given layer thickness. Furthermore, thesol-gel layers can be used as protective or passivation layers whichprotect the reflector surfaces against weathering or corrosion.

Further sol-gel layers contain e.g. polymerisation products fromorganically substituted alkoxy-siloxanes having the general formula;

    Y.sub.a Si(OR).sub.4-n

where Y is e.g. a non hydrolisable monovalent organic group and R ise.g. an alkyl, aryl, alkaryl or aralkyl group and n is a natural numberfrom 0 to 3. If n is equal to 1 or 2, R may be a C₁ -C₄ alkyl group. Ymay be a phenyl group, n equal to 1 and R a methyl group.

In another version the sol-gel layer may be a polymerisation product oforganically substituted alkoxy-compounds having the general formula:

    X.sub.n AR.sub.4-n

where A represents Si, Ti, Zr or Al, X represents HO--, alkyl-O-- orCl--, R represents phenyl, alkyl, alkenyl, vinylester or epoxyether andn the number 1, 2 or 3. Examples of phenyl are unsubstituted phenyl, ormono- , di- or tri-substituted C₁ -C₉ -alkyl-substituted phenyl, foralkyl, equally methyl, ethyl, propyl, iso-propyl, n-butyl, pentyl etc.,for alkenyl --CH═CH₂, allyl, 2-methylallyl, 2-butenyl etc., forvinylester --(CH₂)₃ --O--C(═O)--C(--CH₃)═CH₂ and for epoxy-ether--(CH₂)₃ --O--CH₂ --CH(--O--)CH₂.

Such sol-gel layers with polymerisation products of organicallysubstituted alkoxyl compounds are, to advantage, deposited by a sol-gelprocess directly or indirectly onto the surface of the reflector body oronto the free surface of the LI layer. For that purpose e.g. alkoxidesand halogen-silanes are mixed and hydrolised and condensed in thepresence of water and suitable catalysts,. After removing the water andthe solvent, a sol forms and may be deposited on the surface to becoated by immersion, centrifugal means, spraying etc., whereby the solLransforiis into a gel film e.g. under the influence of temperatureand/or radiation. As a rule silanes are employed to form the sol; it isalso possible to replace the silanes partially by compounds containingtitanium, zirconium or aluminium instead of silicon. This enables thehardness, density and the refractive index of the sol-gel layer to bevaried. The hardness of the sol-gel layer may also be controlled byemploying different silanes e.g. by forming an inorganic network tocontrol the hardness and thermal stability, or by employing an organicnetwork to control the elasticity. A sol-gel layer which may becategorised between inorganic and organic polymers can be deposited onthe surface to be coated via the sol-gel process by hydrolysis andcondensation of alkoxides, mainly those of silicon, aluminium, titaniumor zirconium. In the process an inorganic network is formed andadditionally, via appropriately derivatised silicic acid-esters, it ispossible to incorporate organic groups which may he employed forfunctionalising and for forming defined organic polymer systems.Further, the sol-gel film may be deposited by electro-immersion coatingafter the principle of cateplioretic precipitation of an amine andorganically modified ceramic.

The HI layer may also be a sol-gel layer made up of two or moresab-layers. The HI layer may thereby be made up of a plurality ofsub-layers of different composition and refractive indices. Essential inthat respect is that each sub-layer exhibits a higher refractive indexthan that of the LI layer.

With regard to the thickness of the LI/HI layers, it was found in thecourse of the activities concerning the invention that the properties ofreflectivity run essentially periodically and namely such that withincreasing layer thickness--in particular layers with an optical layerthickness d_(opt),i greater than 6 λ/4--the reflection properties areunsuitable for technical lighting purposes. Preferred therefore arelayers with an optical layer thickness less than 6λ/4 and in particularsuch with l₁ and l₂ equal to 1 or 3.

Also, was found that reflectors with LI/HI layer thicknesses lying inthe thickness range d_(i).n_(i) =l_(i).λ/4±20 nm (i=1 or 2) exhibitessentially the same good reflection properties so that the layerthicknesses d_(i) do not have to comply exactly with the conditiond_(i).n_(i) =l_(i).λ/4, i=1 or 2.

Over the whole of the reflector surface, therefore, the LI and Hi layersexhibit a constant thickness that does not vary by more than ±20 nm. Thethicknesses d₁ of the LI layer and d₂ of the HI layer are preferablybetween 30 and 350 nm, in particular between 40 and 120 nm.

Highly preferred is for the thicknesses d₁ and d₂. measured at anyparticular spot not to differ by more than ±5% from the average layerthicknesses d_(1av) and d₂ av over the whole of the composite layer.

Also preferred is for the thicknesses of the LI and HI layers to beselected such that the optical layer thicknesses of the LI and HI layerssatisfy the equation

    d.sub.opt,i =d.sub.i.n.sub.i =l.sub.i.λ/4±20 nm, i=1 or 2

for a wavelength corresponding to the average wavelength best perceivedby the human eye in daylight conditions, which is approximately 550 nm.The thicknesses of the LI and HI layers are chosen in particular suchthat the above equation holds for their optical layer thicknesses forwavelengths of λ=550 nm ±200 nm.

In order that constructive interference can take place and in order thatthe absorption of the electromagnetic radiation to be reflected is assmall as possible, the composite layer must be as transparent aspossible to the radiation to be reflected and must be pore-free. Thelatter requirement, apart from minimising absorption of light, alsocontributes to avoiding uncontrollable diffuse scattering of light. Bypore-free is not to be understood absolutely pore-free. Important inthat respect is that the porosity of the composite layer is less than1%. The composite layer with such LI/HI layers preferably exhibits anabsorption of incident light energy amounting to less than 3%.

The LI layer is preferably a barrier layer of aluminium oxide formed byanodising or an LI sol-gel layer.

The production of an LI layer of aluminium oxide by way of anodisingrequires e.g. a clean aluminium surface i.e. normally, prior toanodising, the aluminium surface which is to be oxidisedelectrolytically must be subjected to a surface treatment, a so calledpre-treatment.

The aluminium surfaces usually exhibit a naturally occurring oxide layerwhich, frequently because of their previous history etc. is contaminatedby foreign substances. Such foreign substances may for example beresidual rolling lubricant, oils for protection during transportation,corrosion products or pressed in foreign substances and the like. Inorder to remove such foreign substances, the aluminium surfaces arenormally pre-treated chemically with a cleaning agent that produces somedegree of attack by etching. Suitable for this purpose--apart fromaqueous acidic degreasing agents--are in particular alkaline degreasingagents based on polyphosphate and borate. A cleaning action smithmoderate to strong removal of material is achieved by caustic or acidicetching using strongly alkaline or acidic pickling solution, such ase.g. caustic soda or a mixture of nitric acid and hydrofluoric acid. Inthat cleaning process the natural oxide layer is removed and along withit all the contaminants contained in it. When using strongly attackingalkaline pickling solutions, a pickling deposit often forms and has tobe removed by an acidic after-treatment. A surface treatment withoutremoving surface material is a degreasing treatment which may beperformed using organic solvents or aqueous or alkaline cleaning agents.

Depending on the condition of the surface, it may also be necessary toremove surface material using mechanical means. Such a surface treatmentmay be performed e.g. by grinding, surface blasting, brushing orpolishing, and if desired may be followed by a chemical after-treatment.

In the blank metallic state aluminium surfaces exhibit a very highcapacity to reflect light and heat. The smoother the surface, thegreater is the directional reflectivity and the brighter the appearanceof the surface. The highest degree of brightness is obtained with highpurity aluminium and special alloys such as e.g. AlMg or AlMgSi alloys.

A highly reflective surface is obtained e.g. by polishing, milling, byrolling with highly polished rolls in the final pass, by chemical orelectrolytic polishing, or by a combination of the above mentionedsurface treatment methods. The polishing may be performed using clothwheels with soft cloth, if desired using a polishing paste. Whenpolishing with rolls it is possible to introduce a given structure tothe surface of the aluminium using engraved or etched steel rolls or byplacing some means exhibiting a given structure between the rolls andthe material being rolled. Chemical polishing is performed e.g. using ahighly concentrated acid mixture normally at high temperatures of around100° C. Acidic or alkaline electrolytes may be employed for electrolyticbrighitening; normally acidic electrolytes are preferred.

At least the part of the reflector body bearing the aluminium layer tobe oxidised is then placed in an electrically conductive liquid, theelectrolyte, and connected up as the anode to a direct current source,the negative electrode normally being of stainless steel, graphite, leador aluminium.

The electrolyte is made such that, during the anodising process, thealuminium oxide formed does not re-dissolve, i.e. no re-solution of thealuminium oxide takes place. In the dc field, gaseous hydrogen forms atthe cathode and gaseous oxygen at the anode. The oxygen forming at thealuminium surface reacts with the aluminium and forms an oxide layer onit which becomes increasingly thicker in the course of the process. Asthe resistance of the layer increases rapidly with increasing thicknessof the barrier layer, the flow of current drops accordingly and thegrowth of the layer comes to a halt.

Manufacturing LI and layers electrolytically enables the layerthicknesses to be controlled precisely. The maximum thickness of thealuminium oxide barrier layer corresponds approximately in nanometres(nm) to the voltage in volts (V) applied i.e. the maximum thickness oflayer obtained is a linear function of the anodising voltage, wherebythe voltage drop in the outer layer has to be taken into account. Theexact value of the maximum layer thickness obtained as a function of theapplied direct voltage U, taking into account the voltage drop in theouter layer, can be determined by a simple trial and lies between 1.2and 1.6 nm/V, whereby the exact value of layer thickness as a functionof the applied voltage depends on the electrolyte employed i.e. itscomposition and temperature.

Accordingly the minimum applied voltage U_(min) in volts is:

    d.sub.1 /1.6≦U.sub.min ≦d.sub.1 /1.2

where d₁ represents the thickness in nm of the LI layer with refractiveindex n₁ which has to satisfy the following relationship

    d.sub.1.n.sub.1 =l.sub.1.λ/4±20 nm

In order to take into account the change in voltage drop as a functionof time, the applied anodising voltage may be raised continuously or insteps throughout the anodising process. The optimum anodising voltage orthe optimum change in anodising voltage and the duration of anodisingmay be determined in a simple trial beforehand or via reflectivitymeasurements made during the anodising process.

The electrolytic oxidation may be carried out in a single process stepby applying a predetermined anodising voltage, or continuously or in aseries of steps in which the anodising voltage is raised to apredetermined level or to a level which is determined by measuring theoptimum reflectivity characteristics. The electrolytic oxidation play,however, he performed in a series of steps i.e. in a series of processsteps, e.g. employing different anodising voltages.

Preferred is a process in which the reflectivity characteristics of thecomposite layer ate measured continuously and the anodising voltage U involts, starting from U_(A) the initial voltage according to therelationship

    d.sub.1 /1.6≦U.sub.A ≦d.sub.1 /1.2

is increased continuously or in a series of steps until the measuredreflectivity has reached a desired minimum.

By using a non-redissolving electrolyte, the aluminium oxide barrierlayers are almost pore-free, i.e. any pores resulting e.g. fromcontaminants in the electrolyte or structural faults in the aluminiumsurface layer, but only insignificantly due to dissolution of thealuminium oxide in the electrolyte.

LI layers manufactured in this manner have an exactly prescribed layerthickness, are pore-free, homogeneous and transparent to electromagneticradiation, especially that in the visible and /or infra-red range.

Useable as non-redissolving electrolytes in the process according to theinvention are e.g. organic or inorganic acids, as a rule diluted withwater, having a pH of 2 and more, preferably 3 and more, especially 4and more and 7 and less, preferably 6 and less, especially 5.5 and less.Preferred are electrolytes that function cold i.e. at room temperature.Especially preferred are inorganic or organic acids such as sulphuric orphosphoric acid at low concentration, boric acid, adipinic acid, citricacid or tartaric acid, or mixtures thereof, or solutions of ammonium orsodium salts of organic or inorganic acids., especially the mentionedacids and mixtures thereof. In that connection it has been found thatthe solutions preferably contain a total concentration of 20 g/l orless, usefully 2 to 15 g/l of ammonium salt or sodium salt dissolved inthe electrolyte. Especially preferred are solutions of ammonium salts ofcitric or tartaric acidic or sodium salts of phosphoric acid.

A very highly preferred electrolyte contains 1 to 5 wt. % tartaric acidto which may be added e.g. an appropriate amount of ammonium hydroxide(NH₄ OH) to adjust the pH to the desired value.

The electrolytes are as a rule aqueous solutions.

The maximum possible anodising voltage is determined by the dielectricstrength of the electrolyte. This is dependent for example on theelectrolyte composition and temperature, and normally lies in the rangeof 300 to 600 V.

The optimum electrolyte temperature for the anodising process depends onthe electrolyte employed--is, however, of lesser importance for thequality of the LI layers obtained. Temperatures of 15 to 40° C.,especially 18 to 30° C., are employed for the anodising process.

The dielectric constant ε₁ of such an LI layer depends, among otherthings, on the process parameters employed during anodising. Thedielectric constant ε₁ of the LI layer according to the invention liesbetween 6 and 10.5 at 20° C., preferably between 8 and 10.

The aluminium oxide barrier layer acting as LI layer usefully has arefractive index n₁ between 1.55 and 1.65.

In a further preferred version the LI layer is a sol-gel layer. The LIsol-gel layer is preferably comprised of or contains aluminium oxide orsilicon oxide. In another version the LI sol-gel contains or iscomprised of one of the oxides already mentioned for the HI sol-gellayer, the sol-gel material for the LI layer being selected such thatthe difference in refractive index is as great as possible with respectto that of the HI layer.

The LI layer may also be a sol-gel layer comprising two or moresub-layers. Thereby, the LI layer may also be made up of a plurality ofsub-layers differing with respect to composition and refractive index.Essential in that respect is that each sub-layer exhibits a lowerrefractive index than that of the HI layer.

The present invention includes also reflectors with a compositereflectivity-enhancing layer in which the compositereflectivity-enhancing layer is built up of a plurality of LI/HIcomposite layers lying on top of each other, the various LI/HI compositelayers each being made up of the same materials with the same refractiveindices or, the composite reflectivity-enhancing layer may feature aplurality of LI/HI composite layers of different materials with variousrefractive indices.

An advantage of reflectors with a sol-gel layer as LI layer over thosewith an LI oxide layer produced by anodising is that the chemicalcomposition of the reflector body is unimportant, i.e. reflector bodiesof impure and cheap materials may be employed. Further, the choice ofreflector body materials is not restricted by their ability to beanodised, i.e. for example one could also employ plastics as reflectorbody material.

Further advantageous forms of reflectors according to the invention aredescribed in the subclaims.

The reflectors according to the invention find preferred use intechnical lighting applications or as reflectors for infra-red oruv-radiation. A highly preferred application for the reflectorsaccording to the invention is in lamps for lighting technologyespecially for daylight lighting applications, especially functionallamps such as lamps in work places using computer screen monitors,secondary lighting lamps, scanning lamps or as lighting elements,illuminated ceilings or light deflecting channels.

The process according to the invention relates to the above describedproduction of reflectors with an LI layer in the form of a barrier layermade by anodising and a subsequently deposited sol-gel layer as HIlayer. The corresponding process is suitable for producing areflectivity-enhancing composite layer either continuously orindividually on strips, sheets, foils or parts made of aluminium, and oncomposite materials having, at least one outer layer of aluminium.

A further process relates to the production of thereflectivity-enhancing composite layer in a continuous strip process,e.g. using a continuous production line. The continuous production linecomprises essentially either of a strip anodising unit for producing aLI barrier layer by anodising and a coating unit for producing the HIsol-gel layer, or else of a single or multiunit coating line for LI andHI sol coating with after treatment facilities for transforming the sollayers to gel layers.

The LI and HI layers of the composite layer according to the inventionare of only small thickness so that the variation in thickness comparedto the wavelength of incident electromagnetic radiation is very small;consequently any selective absorption of light or irridescence isnegligible.

FIG. 1 is a schematic of the cross-sectional structure of the reflectorof the invention.

In FIG. 1, the reflector 10 includes a reflector body 14 of metal and acomposite reflectivity-enhancing layer 20 facing the surface 16 of thereflector body 14 and consisting of an outer HI-layer 24 with arefractive index n₂ and an outer surface 26 facing the radiation to bereflected and between the reflector body 14 and the HI-layer 24 anLI-layer 22 with a refractive index n₁ which is smaller than n₂. TheLI-and HI-layers are λ/4-layers and the HI-layer 24 is a sol-gel layercontaining or consisting of a polymerization product of organicallysubstituted alkoxy compounds. Referring to FIG. 1, d₁ represents thethickness of the LI-layer 22 and d₂ represents the thickness of theHI-layer 24.

EXAMPLE

A bright aluminium surface of 99.90 wt% Al is provided with a compositereflectivity-enhancing layer; its reflectivity properties are comparedwith the surface properties of a bright aluminium surface having only adc/H₂ SO₄ -oxide layer (an oxide layer produced by anodising with directcurrent in a sulphuric acid electrolyte).

The following table shows the comparison of the typical reflectivityproperties, especially the respective fractions of directional andscattered reflected radiation. Shown in the first column are the valuesobtained with a strip anodised aluminium surface having an approximately1.5 to 2.0 μm dc/H₂ SO₄ -oxide layer; the second column shows the valuesobtained with a bright aluminium surface having an approximately 80 nmthick barrier layer on which an additional, sol-gel layer, essentiallyof TiO₂ has been deposited. Listed in the third column are the valuesobtained with a bright aluminium surface which has an approx. 120 nmthick LI sol-gel layer containing essentially SiO2 and an HI sol-gellayer containing essentially TiO₂. The HI sol-gel layer containingessentially TiO₂ is produced by depositing a titanium-butylate solutionas a sol. the values of directional reflectivity are obtained bysubtracting the scattered radiation from the total reflectivity. Thebright aluminium surfaces are of aluminum having a purity of 99.90 wt %.The surface quality of the bright aluminium surface is the same for allthree types of reflector. The reflectivity values in table 1 wereobtained according to DIN 5036; each represents a technical lightingvalue i.e. the measured reflectivity values are expressed in terms oflight sensitivity of the human eye. As can be seen from the listedvalues, the total reflectivity and the directional reflectivity are bothimproved by the reflectivity-enhancing composite sol-gel layersaccording to the invention. Table 1 also shows details of the opticalquality of the reflector surfaces detected by the eye viz., with regardto streakiness and iridescence; these details show e.g. that anyunevenness present on the surface is smoothed out by applying thesol-gel layers according to the invention.

    __________________________________________________________________________             Al 99.9/045/AN                                                                Strip-anodised                                                                         Al 99.9/Barrier layer                                                                   Al 99.9/Sol-gel(SiO.sub.2)                                 dc/H.sub.2 SO.sub.4 layer;                                                             Sol-gel (TiO.sub.2)                                                                     Sol-gel (TiO.sub.2)                               __________________________________________________________________________    Total reflection                                                                       88%      93%       95%                                               Scattered reflection                                                                    7%       5%        5%                                               Streakiness                                                                            visible  none      none                                              Iridescence                                                                            visible  none      none                                              __________________________________________________________________________

What is claimed is:
 1. A reflector having a composite reflectivity-enhancing layer as a reflecting surface layer on a reflector body where the reflector body is a metal, where the said composite layer has an outer layer facing the radiation to be reflected, the HI-layer, with a refractive index n₂, and, between the reflector body and the outer layer, an LI-layer with a refractive index n₁ which is smaller than n₂ and the LI and HI layers are λ/4 layers, the HI layer is a sol-gel layer containing or consisting of a polymerization product of organically substituted alkoxy compounds and the optical layer thickness d_(opt),1 of the LI layer and d_(opt),2 of the HI layer are such that:

    d.sub.opt,l =d.sub.l.n.sub.l =l.sub.l.λ/4±20 nm, l=1 or 2

where d₁ represents the thickness of the LI layer in nm, d₂ the thickness of the HI layer in nm, λ the average wavelength in nm of the light striking the reflector surface and l₁, l₂ are uneven natural numbers, the thickness d₁ of the LI layer and the thickness d₂ of the HI layer are each between 30 and 350 nm, and are such that the thickness d₁ and d₂ measured at a particular site does not vary by more than ±5 percent from the average values of d_(1av) and d_(2av) over the whole of the composite layer.
 2. The reflector according to claim 1, wherein the quotient n₁ /n₂ lies between 0.5 and 0.7.
 3. The reflector according to claim 1, wherein the composite reflectivity-enhancing layer has a porosity of less than 1 percent.
 4. The reflector according to claim 1, wherein the composite layer absorbs less than 3 percent of the incident light energy.
 5. The reflector according to claim 1, wherein the reflector body or at least the surface layer of the reflector body that has to be provided with the composite layer is of aluminum or aluminum alloy.
 6. The reflector according to claim 5, wherein the aluminum is pure aluminum having a purity of 98.3 weight percent Al and higher.
 7. The reflector according to claim 5, wherein the aluminum is an aluminum alloy having an aluminum content of 98.3 weight percent and higher and containing at least one of the elements Si, Mg, Mn, Cu, Zn and Fe.
 8. The reflector according to claim 5, wherein the LI layer is a transparent and pore-free barrier layer produced by anodizing the aluminum layer and having a dielectric constant ε₁ of 6 to 10.5 at 20° C.
 9. The reflector according to claim 1, wherein the LI layer is a sol-gel layer and comprises a silicon oxide, aluminum oxide or an oxide of an alkali, alkaline earth or transition metal, a lanthanide or an alloy of these materials, or mixtures of the mentioned oxides, or a metal fluoride.
 10. The reflector according to claim 1, wherein the LI layer is a sol-gel layer and is a polymerization product of organically substituted alkoxy compounds.
 11. A process comprising using the reflectors according to claim 1 as reflectors for lamps for technical lighting applications.
 12. A process comprising using the reflectors according to claim 1 as reflectors for lamps for daylight technical lighting applications.
 13. A process comprising using the reflectors according to claim 1 as reflectors for infra-red radiation.
 14. A process comprising using the reflectors according to claim 1 as reflectors for UV radiation.
 15. A process comprising using the reflectors according to claim 1 as reflectors for secondary lighting lamps.
 16. A process comprising using the reflectors according to claim 1 as reflectors for lamps operated in conjunction with computer screen monitors in work places.
 17. A process comprising using the reflectors according to claim 1 as reflectors for scanning lamps.
 18. A process comprising using the reflectors according to claim 1 as reflectors for lighting elements.
 19. A process comprising using the reflectors according to claim 1 as reflectors for illuminated ceilings.
 20. A process comprising using the reflectors according to claim 1 as reflectors for light deflecting channels.
 21. A process comprising using the reflectors according to claim 1 for reflecting electromagnetic radiation with a wavelength λ corresponding to the average wavelength of visible light best perceived by the human eye in daylight. 